1. Field of the Invention
The present invention relates to a wiring board such as a tape carrier substrate formed by providing a conductive wiring on a flexible insulating base. Particularly, the present invention relates to a structure of a wiring board having conductive wirings whose folding resistance is improved.
2. Description of Related Art
As one type of package module using a tape carrier substrate, the one employing a COF (Chip On Film) has been known. The COF includes a semiconductor chip mounted on a flexible insulating tape carrier substrate and the mounted portion is protected by sealing with a resin. The tape carrier substrate includes as main components an insulating film base and a plurality of conductive wirings formed on a surface of the film base. In general, polyimide is used as a material of the film base and copper is used as a material of the conductive wirings. As required, a metal plating film and a solder resist layer as an insulating resin are formed on the conductive wirings.
The COF is used mainly for packaging a driver for driving a display panel such as a liquid crystal panel. In such a case, conductive wirings on the tape carrier substrate are arranged in two groups of a first group for forming external terminals for output signals and a second group for forming external terminals for input signals, and a semiconductor chip is packaged between the first group of conductive wirings and the second group of conductive wirings. The first group of conductive wirings forming the external terminals for output signals are connected to electrodes formed in the periphery of the display panel, and the second group of conductive wirings forming the external terminals for input signals are connected to terminals of a mother board.
On the other hand, a packaging method of folding the tape carrier substrate at its end portion is used for providing a smaller structure to be packaged in the display panel. For facilitating folding at an acute angle, it is known to previously form a slit at the portion of the film base to be folded so as to fold only a copper foil lead that forms the conductive wirings.
However, in the structure, since the film base is not provided at the folded portion where the slit is formed, mechanical stress during the packaging or stress caused by a heat cycle or the like will be concentrated to the conductive wirings made of a thin copper lead, and cracks and ruptures in the conductive wirings can occur easily. Particularly, in a case of Sn plating or Sn-Pb plating both of which are provided most widely on conductive wiring, a brittle Cu-Sn diffused alloy layer is formed on the interface between the copper foil lead and the plating due to the heat applied during the package assembly process, and thus the folding resistance of the conductive wirings is degraded considerably.
A structure for improving the folding resistance of the conductive wirings is described in JP H05-326643 A, for example. In this structure, either Sn or Sn alloy is not plated on the surface of the copper foil lead positioned on a folding slit formed on a film base, and furthermore, a flexible resin film (solder resist) having a plating resistance and insulation is provided on the surface of a copper lead positioned on the folding slit.
However, even in the wiring board having the above-mentioned conventional structure, folding resistance of the conductive wirings made of a copper lead cannot be sufficient anymore. The reason is that, in accordance with the trend for smaller packages using tape carrier substrates, thicknesses for all the components have been reduced. For example, the thickness of the film base of polyimide is about 40 μm, the thickness of the conductive wirings is about 8 μm, and the thickness of the solder resist is about 15 μm. As a result, even at the time of folding the conductive wirings with the film base, stress being not much different from the case of folding the conductive wirings alone will be applied, so that the conductive wirings can be broken easily. In light of this, there is a demand for a structure for relieving sufficiently the bending stress applied to the conductive wirings.